You should do lots of plyometrics for explosivness and do things like squats and push-press. Also hit the body bag and speed bag often, speed is power too. Also when you punch open up your chest and exzagerate the motions when practicing if u know what i mean.
peace

In my opinion u have to be born with heavy hands. Other than that the best u can do is learn how to punch properly using leverage and your weight.Tito for example fights with his right foot on his tip toes,so he puts his weight on his right hand punch using his calf muscle. His left hook comes out of leverage throwing his weight using his thigh muscles making this his harder punch.

Also make sure you stop moving when you throw your punches. To sit down on your punches you have to use your whole body. Everyone is right about technique being the most important thing. A lot of power comes from the whole body twisting motion. Plant your feet, throw your whole body into it and make sure to aim past your target.